1. Field of the Invention
The present invention relates to a variable magnification optical system, particularly a variable magnification optical system which can be downsized. The present invention also relates to an image pickup device including this variable magnification optical system and a digital apparatus incorporating this image pickup device.
2. Description of the Background Art
In recent years, there has been a remarkable increase in the use of mobile terminals provided with a mobile communication function such as mobile phones and PDAs (Personal Digital Assistants), and compact image pickup devices capable of still image recording and video recording such as digital still cameras and digital video cameras are built in these apparatuses in many cases. The image pickup devices incorporated in these apparatuses normally have strict size (volume) and cost restrictions. Thus, image pickup devices using a smaller image pickup element with a smaller number of pixels as compared with standalone so-called digital cameras and including a single-focus optical system composed of about one to three plastic lenses have been generally adopted. However, the image pickup devices incorporated in these apparatuses have also come to include more pixels and become more functional. Thus, there is a demand for a compact variable magnification optical system compatible with a high-pixel image pickup element and capable of recording an object distant from a photographer at a zoom ratio exceeding about 2.5× and being incorporated into these apparatuses.
A variable magnification optical system disclosed in Japanese Unexamined Patent Publication No. 2006-098962 (D1) can be, for example, cited as such a variable magnification optical system. The variable magnification optical system disclosed in this document D1 is a four-component optical system of negative-negative-positive-negative arrangement designed to be compact for a so-called digital camera, but is not sufficiently downsized in being incorporated into the above apparatuses such as the mobile phones.